The invention relates to improvements in methods of and apparatus for testing visual functions of human eyes, and more particularly to improvements in methods of and apparatus which employ perimeters.
Perimetric examination of a subject's eye involves the determination of sharpness of visual perception on the retina. To this end, and as disclosed in the patent to Billeter et al., the eye to be examined is maintained in a predetermined position and the permeter presents to the eye a plurality of stimuli at a plurality of selected locations in the area of the optical axis of the eye to be examined. The various locations can constitute a raster in a plane which is normal to the optical axis of the eye. The perimeter presents to the eye stimuli of predetermined value (intensity), and the patient acknowledges the detection or discernment of a stimulus by actuating a knob or a like device. The test is thereupon repeated with a sequence of stimuli having different values; such stimuli can be presented to the eye at each of the predetermined locations which were selected for the first test or at a selected number of such predetermined locations. The second test is followed by a third test which presents to the eye stimuli having a third value, and so forth. The testing operation is terminated when the perimeter or the person in charge ascertains (either empirically or by calculation) a series of threshold values of stimuli which are discerned by the eye of the subject at the selected locations in the aforementioned plane that crosses the optical axis of the eye.
A drawback of the aforedescribed conventional methods and apparatus is that the completion of each test takes up a substantial amount of time. Moreover, the examination is tiresome to the subject because the subject must cause the initiation of each and every signal to indicate whether or not a stimulus of a particular value has been perceived by the eye under test. The reliability of a subject's reaction decreases as the testing operation proceeds so that the final results of the examination are often misleading.
In order to shorten successive tests or stages of a complete examination and to thus enhance the accuracy of the examination, it is already known to correct previously gathered and stored information pertaining to the perception of stimuli at some or all of the aforediscussed locations in the area of the optical axis of the eye which is being tested. The correction involves modifying the previously obtained threshold values in dependency on the more recently ascertained threshold values and storing the thus modified threshold values of stimuli for utilization in the course of a later examination of the same eye. Such examination at a later time can include presenting to the eye stimuli of a value corresponding to the previously ascertained and stored threshold values for some or all of the locations. It has been found that such improved method also exhibits a number of drawbacks, particularly as concerns the duration of an examination. Thus, it is still necessary to present a plurality of stimuli having different values at least to a plurality of different locations in the area of the optical axis of the eye which is being tested. The new threshold value for each test location must be ascertained by bracketing in a number of successive stages comprising visible and invisible stimuli which prolongs the examination and is tiresome to the subject in addition to adversely influencing the accuracy of the tests. The just outlined method involves applying to each of several different portions of the retina a plurality of stimuli in order to ascertain a stimulus just below the limit of perception and a stimulus just above such limit. These stimuli are thereupon averaged to ascertain the median value or another mathematical function of the two stimuli. Such procedure is called bracketing the sought-after average value of stimuli for a particular part of the retina, i.e., bracketing renders is possible to ascertain, in accordance with heretofore known methods, a series of stimuli--each for a different part of the retina--each of which is somewhere between a still imperceptible stimulus and a barely perceptible stimulus.